According to the Department of Energy, 21 billion gallons of co-produced water are drawn up by oil and gas wells each year in the United States. Natural “oil” from a well is actually a multiphase fluid of oil/water/gas. Generally all three fluids are found in every hydrocarbon well and well effluent.
Because of its value and because of environmental concerns, oil needs to be separated from this effluent. This is usually done through gravitational settling in large tanks, which requires capital and significant space that is not always available onsite. Gas is separated easily in a mechanical separator or by pressure reduction within storage containers. In the case of heavy oils and many emulsified fluid systems, the raw fluids are heated to change the density of the oil and water by heating off lighter ends and essentially agitating their molecular structures so that these fluids can more easily separate. Water then is a byproduct.
While co-produced water is common, it is a hazardous waste that is very burdensome to operators and must be disposed of, sometimes with difficulty. Water that historically may have sent down a creek or into a wooden barrel now has to be piped into very large storage containers. Offshore, those containers are attached to topsides or hooked by hoses to older, refitted storage ships. This waste water is stored for days to separate oil from water. The resulting aqueous product still contains some oil and other possible contaminants. Not only is this procedure costly, it is time consuming, risky, and energy consumptive. In an offshore subsea environment, water storage containers are not feasible.
Current topsides water-oil separation processes are slow, environmentally unfriendly, and consume a large amount of energy in heating. The final residual water must be disposed, and cannot be released directly into the ocean. Stored water often must be treated by chemicals to reduce phase surface tensions and induce separation, creating further risk from chemical handling and possible spills. Volumes of produced water offshore can be in the thousands of barrels per day per well. Processing facilities generally are a large part of an ongoing footprint. Co-produced water is frequently re-injected into the subsurface because topsides storage is a non-trivial issue. Sufficient removal of controversial chemicals and residual oil before any disposal is a process concerning regulators, such as the U.S. Environmental Protection Agency (EPA).
In subsea operations, as noted above, there is simply no large volume water storage option available to industry. Separation is ineffective: a significant amount of water-in-oil remains in solution after subsea separation, resulting in inefficient separation and ultimately in lower hydrocarbon recoveries. More critically, oil-in water concentrations are high (in the range of 5 percent) because of a lack of sufficient residence time, which will result in crude oil injection into disposal wells along with produced water. Consequently, the disposal zones eventually plug and become ineffective for additional disposal. Additionally, the oil-in-water that is pumped into disposal zones is lost product and revenue to both the operating companies and to the Federal government from royalties.
It would be desirable to provide a simple, inexpensive technique that has a very small and lightweight footprint which can more effectively separate oil from its reservoir water in real-time without using large settling tanks. Such a technique can save the industry hundreds of millions of dollars in operating and maintenance costs and increase revenues besides saving space. Additionally, that method can eliminate many environmental concerns.